The solar cell is a well known device for converting sunlight into electrical energy. Such solar cells comprise a silicon wafer having a number of p- and n-doped regions which together generate a potential difference and current when exposed to sunlight.
In addition to the base silicon wafer, solar cells typically comprise electrical contact structures for collecting the electricity from the doped regions. Those structures typically include one or more layers of metal, together with insulating layers and passivating layers, and often an uppermost layer of a solderable metal to give good solder contact with a support frame. Solar cells are described in, for example, U.S. Pat. No. 6,333,457, US 2004/0200520 and U.S. Pat. No. 6,337,283.
The manufacture of solar cells, and in particular the creation of the electrical contact structures, typically involves multiple stages of electroplating and/or etching. For example, it may be necessary to etch away specific areas of a silicon dioxide passivating layer in order to allow a subsequently deposited metal layer to make contact with doped regions of the wafer, or it may be necessary to deposit a conductive metal such as copper in a specific pattern so that it contacts, say, the p-doped regions but not the n-doped regions. One method of selectively etching and plating predetermined regions of the wafer is to apply a protective masking material, known as a resist, to those areas which are not to be etched or plated, thereby preventing etching or plating in those regions, subjecting the wafer to the etching or plating process, and then removing the resist, typically by spraying or washing with aqueous alkali.
Conventionally, the resists are applied to the wafer using screen printing. That technique gives good image definition and is reliable. The screen printing of resists during solar cell manufacture is described in, for example, WO 2005/013323 and WO 2005/011979.
Recently, there has been a move to using thinner, more delicate silicon wafers, and to make electrical contact structures having a finer level of detail. Accordingly, there is a need for an improved method of etching and/or plating.